1. Field of the Invention
The present invention relates to a fixing device which heats a heating object through induction heating by using a plurality of induction heating coils.
2. Description of the Related Art
Generally, electrophotographic image forming devices according to the related art are provided with a fixing device which is adapted to fix toner to paper. Induction heating (IH) is known as one of various heating methods used in such a fixing device. Generally, induction heating (IH) is the process of heating an electrically conducting object by electromagnetic induction in which eddy currents are generated within the object and resistance leads to Joule heating of the object. In a power device (inverter) for induction heating, the amount of an alternating current (AC) that flows through an induction heating coil is controlled according to an operating frequency (ON time) of a switching element arranged in the inverter. Namely, it is known that the power control of the IH fixing system is carried out through the ON-time control of the switching element.
Moreover, in image forming devices according to the related art, recording sheets of various sizes (sheet widths) are used depending on the respective specifications of the image forming devices. For example, see Japanese Laid-Open Patent Publication No. 2000-206813, Japanese Laid-Open Patent Publication No. 2001-312178, and Japanese Patent No. 4,021,707. As disclosed in these publications, it is known that the image forming devices are provided with a plurality of induction heating coils which are arranged in parallel with the axial direction of a heating roller to have an arrangement distance suitable for the recording-sheet width, and the power supplied to the induction heating coils is controlled.
Further, in the image forming devices according to the related art, when supplying the power to each of the induction heating coils arranged in parallel with the axial direction of the heating roller, the electric energy generated in each of the induction heating coils is changed according to the temperature difference in the longitudinal direction of the heating roller. This means that there are two or more induction heating coils which are driven at different frequencies. In this case, interference noise may arise.
It is known that the timings of the power supply to the coils are to be changed in order to prevent the occurrence of the interference noise. However, if the power distribution ratios of the coils are maintained and the power supplying timings are changed to prevent the occurrence of the interference noise, the coils will be heated in a time-division manner with the changed supplying timings. In such a case, the fixing device according to the related art must have an extended time for reaching the fixing permissible temperature.